This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to JP2002-193665 filed in Japan on July 2, 2002; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an ion attachment mass spectrometry apparatus, ionization apparatus, and ionization method, more specifically relates to an apparatus able to analyze the mass of a sample gas at a high sensitivity without causing disassociation of molecules of the sample gas and an ionization apparatus and ionization method suitable for that apparatus.
2. Description of the Related Art
A mass spectrometry apparatus for measuring the molecular weight of a sample gas passes an ionized sample gas through an electromagnetic field (one or both of an electric field and magnetic field) to separate it by mass and detect the weight. The electron impact method, the most general of the ionization methods, causes electrons to strike the sample gas at a high energy of about 70 eV and uses the impact energy to strip electrons from the molecules of the sample gas to obtain positive ions. However, according to the electron impact method, there was the problem that the molecules of the sample themselves are split (disassociated) by the high impact energy and therefore correct measurement was not possible.
Therefore, the ion attachment method has been developed as a method for ionization of molecules of a sample gas without causing disassociation. This ion attachment method has been reported in Hodges, Analytical Chemistry, vol. 48, no. 6, p. 825 (1976); Bombick, Analytical Chemistry, vol. 56, no. 3, p. 396 (1984); Fujii et al., Analytical Chemistry, vol. 61, no. 9, p. 1026 (1989), Chemical Physics Letters, vol. 191, no. 1.2, p. 162 (1992), Japanese Unexamined Patent Publication (Kokai) No. 6-11485, and Japanese Examined Patent Publication (Kokoku) No. 7-48371.
In the ion attachment method, first, an emitter including a metal salt of Li, Na, Al, etc. is heated to cause the generation of metal ions such as Li+, Na+, and Al+. Next, the metal ions are brought into contact with the sample molecules, whereupon the metal ions attach to locations where the charges of the sample molecules concentrate and the sample molecules as a whole become ions (hereinafter called xe2x80x9cattached ions or pseudo-molecule ionsxe2x80x9d). The energy of attachment of the metal ions to the sample molecules, that is, binding energy, is an extremely small one of about 1 eV. This is smaller than the normal binding energy of compounds of 2 to 3 eV, so the molecules will not easily disassociate even after attachment.
However, if the surplus energy remains in the above attached ions, the metal ions with the surplus energy will disassociate and in turn the sample gas will return to its original neutral molecules. Therefore, by making the attached ions and atmospheric gas collide, the surplus energy is quickly removed and stable attached ions are obtained. The atmospheric gas may be the sample gas itself or a gas other than the sample gas, but a pressure of about 100 Pa is required. If below 100 Pa, the number of frequency of collisions is small and surplus energy cannot be sufficiently removed.
A mass spectrometry apparatus using the ion attachment method is called an xe2x80x9cion attachment mass spectrometry apparatusxe2x80x9d. The overall configuration of a conventional ion attachment mass spectrometry apparatus is shown in FIG. 17. As shown in this figure, an ion attachment mass spectrometry apparatus is usually comprised of a first chamber 102 provided with an emitter 101 for emitting ions, a second chamber 103 comprising an intermediate chamber, and a third chamber 105 provided with a mass spectrometer 104 for mass spectrometry. The first chamber 102 and second chamber 103 are provided between them with a partition 107 having an aperture 106 of a diameter of about 1 mm is provided between the first chamber 102 and the second chamber 103. The aperture 106 is normally given by a nozzle structure. An aperture 108 is provided between the second chamber 103 and third chamber 105. By evacuation by a vacuum pump, the first chamber 102 is reduced to pressure of 100 Pa, the second chamber 103 to 0.1 Pa, and the third chamber 105 to 10xe2x88x923 Pa or so. Note that the gas 109 introduced into the first chamber 102 may be comprised of the sample alone or may be comprised of mixed gas comprising a base gas such as an inert gas and sample gas. In FIG. 17, details of the configuration of the emitter 101 are omitted.
On the other hand, for an object different from that of an ion attachment mass spectrometry apparatus, there are an inductively coupled plasma (ICP) mass spectrometry apparatus and atmospheric pressure ionization (API) mass spectrometry apparatus, which can measure a sample gas at an extremely high sensitivity. These mass spectrometry apparatuses are provided with first chambers, second chambers, and third chambers similar to those explained above. In both cases, the pressure of the first chamber for ionization is made 1xc3x97105 Pa (atmospheric pressure), the pressure of the second chamber is made 10 to 1000 Pa, and the pressure of the third chamber for mass spectrometry is made 10xe2x88x923 Pa or so.
As a means for ionization, an inductively coupled plasma mass spectrometry apparatus uses plasma, while an atmospheric pressure ionization mass spectrometry apparatus uses a corona discharge. In both cases, the electrons generated are made to collide with the sample gas by an energy of several tens of eV to strip off electrons from the sample molecules and obtain positive ions, then ion exchange or another ionization reaction is caused in a chain to realize highly efficient ionization.
In general, when the pressure is high, the number of collision frequency increases, the chain reaction proceeds faster, and the plasma spreads the ionization reaction by itself (self expansion action), so low ion mobility due to the high pressure does not become a problem. Therefore, in all of the above conventional mass spectrometry apparatuses, the optimal pressure of the first chamber is the atmospheric pressure. Normally, a nozzle having an aperture of a diameter of about 1 mm is provided between the first chamber and the second chamber. Since the first chamber is a high pressure, the gas blown out from the nozzle forms a supersonic jet. This supersonic jet causes the ionized sample to be efficiently transported to the mass spectrometer.
In the ordinary vacuum state, a gas spreads uniformly randomly. The translation energy (speed) of this movement is a thermal motion energy at room temperature, so is 0.03 eV or so. As opposed to this, the supersonic jet is extremely characteristic and is comprised of an xe2x80x9cexpansion partxe2x80x9d, a xe2x80x9csilent partxe2x80x9d, a xe2x80x9cMach diskxe2x80x9d, and a xe2x80x9cbarrel shockxe2x80x9d (see FIG. 2).
The xe2x80x9cexpansion partxe2x80x9d is the part forming a peak of pressure higher than the surroundings near the nozzle outlet. Therefore, the gas or ions collide at a high frequency, a rapid drop in pressure and expansion of gas flow arises, and the gas or ions are cooled by adiabatic expansion. The xe2x80x9csilent partxe2x80x9d is after the expansion part and forms a bowl of pressure lower than the ambient atmospheric gas. The gas or ions proceed forming beams of uniform direction and speed. This thermal energy also reaches about 3 eV or 100 times as high as the thermal energy at room temperature. Note that an inductively coupled plasma mass spectrometry apparatus and atmospheric pressure ionization mass spectrometry apparatus use this characteristic to raise the transport efficiency of ions. The xe2x80x9cMach diskxe2x80x9d is the end of the silent part, while the xe2x80x9cbarrel shockxe2x80x9d is at the side. Both form barriers of pressure higher than the ambient atmospheric gas. The atmospheric gas is blocked by these and cannot penetrate into the silent part.
For the supersonic jet to be formed, it is necessary that the Knudsen number (xcex/D) of the length of mean free path (xcex) of the gas in the first chamber divided by the diameter (D) of the nozzle be less than 0.01 and that the inner pressure of the second chamber is not more than {fraction (1/10)}th of the inner pressure of the first chamber. In particular, if the Knudsen number is not more than 0.001 and the inner pressure of the second chamber is not more than {fraction (1/100)}th of the inner pressure of the first chamber, it is known that a more powerful supersonic jet is formed. An ordinary inductively coupled plasma mass spectrometry apparatus and atmospheric pressure ionization mass spectrometry apparatus satisfy this condition.
Note that with the conventional ion attachment mass spectrometry apparatus explained in FIG. 17, the Knudsen number is about 0.07, so a supersonic jet is not formed.
Note that as an example of use of the characteristic of the xe2x80x9cexpansion partxe2x80x9d of the supersonic jet, the formation of gas clusters is known. Neutral gases are extremely weak in attachment energy with each other, so even if gases collide and temporarily attach to each other, they end up immediately separating due to the surplus energy. Therefore, under ordinary conditions, the gas will never form gas clusters, but in the xe2x80x9cexpansion partxe2x80x9d of a powerful supersonic jet, gas clusters are formed. This is due to two reasons: there are numerous opportunities for attachment since gases collide at a high frequency and surplus energy is quickly removed to cooling by adiabatic expansion.
With a conventional ion attachment mass spectrometry apparatus, there was the problem of a low sensitivity of measurement. Compared with the inductively bonded plasma mass spectrometry apparatus or atmospheric pressure ionization mass spectrometry apparatus, the conventional ion attachment mass spectrometry apparatus has a low sensitivity of 10xe2x88x923 to 10xe2x88x925. This is because (1) the transport efficiency of metal ions to the attachment region, (2) the attachment efficiency of metal ions to the sample gas, and (3) the transport efficiency of attached ions to the mass spectrometer are not sufficient.
FIG. 18 is a detailed enlarged view of the vicinity of an emitter 101 and aperture 106 in a conventional ion attachment mass spectrometry apparatus. The aperture 106 is formed by a nozzle 110. Reference numeral 111 is a jet flow. The Li+ or other metal ions discharged from the emitter 101 and they are repelled each other by the Coulomb force and spread in the four directions in the first chamber 102. However, due to the parallel electric field in the direction of the nozzle 110 and flow of the gas 109, the region 112 where the metal ions are present becomes spherical somewhat toward the nozzle 110. It is not possible to make the metal ions concentrate at a specific region because the length of mean free path at 100 Pa in the first chamber 102 is an extremely short 70 xcexcm and even if making the metal ions move in the electric field, they immediately collide with the gas and end up stopping or changing in direction. On the other hand, since the sample gas spreads uniformly in the first chamber 102, attachment occurs everywhere in the spherical region 112 where the metal ions are present. However, the attached ions generated at a part far from the nozzle 110 cannot reach the nozzle 110, so the effectively used attachment region 113 is limited to a smaller region close to the nozzle 110. Therefore, in a conventional ion attachment mass spectrometry apparatus, the transport efficiency of the metal ions to the attachment region pointed out in the above (1) is not so high.
Next, the attachment region of the metal ions is a constant pressure of 100 Pa, so attached ions are produced by the collision of the randomly moving sample gas and metal ions as thermal motion. After this, the surplus energy is removed by the collision of randomly moving atmospheric gas and attached ions as thermal motion. In each case, since the random motion of the gas due to thermal motion at room temperature is due to the motion of gas, the attachment efficiency of metal ions and sample gas pointed out at the above (2) is not so high.
The attached ions passing through the nozzle 110 are transported to the mass spectrometer 104 by the force of the electric field. However, the attachment ions generated from an attachment region of a certain size pass through the nozzle 110, then have different speeds and directions. With just an electric field, it is difficult to converge and transport ions of different speeds and directions at a specific location. Therefore, the transport efficiency of the attached ions to the mass spectrometer pointed out at the above (3) is not high.
Note that if the first chamber 102 is made to have a higher pressure than 100 Pa, the sensitivity falls. This is because the efficiency of removing the surplus energy becomes saturated at a higher pressure than 100 Pa and no longer increase, while the transport efficiency of the attached ions to the mass spectrometer greatly fall.
The efficiencies of the above (1) to (3) are not sufficient, therefore the sensitivity is low. This is the most important problem in an ion attachment spectrometry apparatus.
Furthermore, in a conventional ion attachment mass spectrometry apparatus, the sample gas contacts the emitter 101, whereby products deposit on the surface of the emitter 101 and the amount of emission of metal ions ends up falling. In particular, in the case of a readily reactable sample gas, this becomes a major problem in practical use.
Further, in a conventional ion attachment mass spectrometry apparatus, there is the problem that the pressure of the measured gas has to be made higher than the pressure of the first chamber 101 (100 Pa). This is because it is necessary to make the pressure higher to pull the sample gas into the chamber. In order to apply this apparatus to broader industrial applications, the measurable gas pressure should be as low as possible.
An object of the present invention is to provide an ion mass spectrometry apparatus improving the transport efficiency of the metal ions to the attachment region, the attachment efficiency of the metal ions and sample gas, and the transport efficiency of the attached ions to the mass spectrometer and raise the measurement sensitivity.
Another object of the present invention is to provide an ionization apparatus and ionization method for attaching metal ions to gas molecules and improving the transport efficiency of the metal ions to the attachment region and the attachment efficiency of the metal ions to sample gas.
The ion attachment mass spectrometry apparatus, ionization apparatus, and ionization method according to embodiments of the present invention are configured as follows to achieve the above objects.
A first ion attachment mass spectrometry apparatus according to a first aspect of the present invention is provided with a first chamber and second chamber separated by a partition having an aperture, an emitter generating positive metal ions, a mass spectrometer, a vacuum pump for reducing the pressure of at least the second chamber, and a sample introduction mechanism for introducing a sample gas. The metal ions are made to attach to the molecules of the sample gas to obtain positive ions and the mass of the sample gas is analyzed by the mass spectrometer. A supersonic jet region is formed in the second chamber by making the Knudsen number (xcex/D, where xcex is the length of mean free path in the first chamber and D is the diameter of the aperture) not more than 0.01, making the pressure of the second chamber not more than {fraction (1/10)}th of the first chamber, and making the gas of the first chamber be blown out from the aperture to the second chamber. The sample gas and metal ions are injected into the supersonic jet region to make the metal ions attach to the molecules of the sample gas at the supersonic jet region.
A second ion attachment mass spectrometry apparatus preferably has a Knudsen number of not more than 0.001, a pressure in the first chamber of at least 1xc3x97105 Pa, and a second chamber of not more than 1xc3x97103 Pa.
A third ion attachment mass spectrometry apparatus preferably gives a relationship between a pressure of the first chamber of P1, a pressure of the second chamber of P2, and a distance from the aperture to the aperture arranged in front of the mass spectrometer of L where L less than 0.67xc3x97Dxc3x97(P1/P2)0.5, whereby the Mach disk of the supersonic jet is positioned behind the aperture.
A fourth ion attachment mass spectrometry apparatus preferably provides an emitter at the first chamber, controls the flow of gas in the first chamber, transports the metal ions generated at the emitter to the vicinity of the aperture inlet of the first chamber, and injects metal ions to the supersonic jet region.
An ionization apparatus according to one embodiment of the present invention is provided with a first chamber and second chamber separated by a partition provided with an aperture, an emitter provided in the first chamber for generating positive metal ions, a vacuum pump for reducing the pressure of at least the second chamber, and a sample introduction mechanism for introducing a neutral gas into the first chamber and causing attachment of metal ions to molecules of sample gas to create positive ions. This ionization apparatus is provided with a supersonic jet region formed in the second chamber by making the Knudsen number (xcex/D, where xcex is the length of mean free path in the first chamber and D is the diameter of the aperture) not more than 0.01, making the pressure of the second chamber not more than {fraction (1/10)}th of the first chamber, and making the gas of the first chamber be blown out from the aperture to the second chamber. Gas and metal ions are injected into the supersonic jet region and metal ions are made to attach to the gas molecules in the supersonic jet region.
An ionization method according to one embodiment of the present invention is a method for ionization by making metal ions attach to neutral gas molecules. It forms two chambers separated by a partition provided with an aperture, introduces gas to one chamber while evacuating the other chamber, makes the Knudsen number (xcex/D, where xcex is the length of mean free path in the first chamber and D is the diameter of the aperture) of not more than 0.01, and gives a pressure difference of at least one order of magnitude in terms of the Pa value between the two chambers so as thereby to form a supersonic jet region in the vicinity of the aperture at the other chamber and injection metal ions into the supersonic jet region for ionization.
Note that in the above ion attachment mass spectrometry apparatus, the following configurations may be adopted:
(1) Providing an emitter in the second chamber, controlling the electric field in the second chamber, and transporting the metal ions generated from the emitter to the vicinity of the aperture outlet of the nozzle of the second chamber so as to inject metal ions in the supersonic jet region.
(2) Providing the emitter in a chamber separated from a first chamber and a second chamber and communicated with the inside of the nozzle, controlling the electric field in the chamber, and transporting metal ions generated from the emitter to the inside of the nozzle so as to inject metal ions into the supersonic jet region.
(3) Making all or part of the nozzle an emitter and generating metal ions from all or part of the inside wall forming the nozzle so as to inject metal ions into the supersonic jet region.
(4) Connecting the sample gas introduction mechanism to the first chamber and transporting the sample gas to the vicinity of the nozzle inlet of the first chamber so as to inject the sample gas into the supersonic jet region.
(5) Connecting the base gas introduction mechanism to the first chamber and connecting the sample gas introduction mechanism to the second chamber and transporting the sample gas to the vicinity of the nozzle outlet of the second chamber so as to inject the sample in the supersonic jet region.
(6) Connecting the base gas introduction mechanism to the first chamber, connecting the sample gas introduction mechanism to a chamber separated into a first chamber and second chamber and communicated with the inside of the nozzle, and transporting the sample gas to the inside of the nozzle so as to inject the sample in the supersonic jet region.
(7) Making gas be blown out from the second nozzle to the second chamber and thereby forming a second supersonic jet region of a supersonic speed at the second chamber under the conditions that the tip of the sample introduction mechanism forms a second nozzle, the Knudsen number (xcexxe2x80x2/Dxe2x80x2) of the length of mean free path xcexxe2x80x2 of the gas in the vicinity of the inlet of the second nozzle divided by the diameter Dxe2x80x2 of the second nozzle is not more than 0.01, and the pressure in the second chamber is not more than {fraction (1/10)}th of the pressure at the vicinity of the inlet of the second nozzle.
In the embodiments of the present invention, the pressures of the first chamber and second chamber and the nozzle having an aperture between these chambers are made to satisfy specific conditions to form a supersonic jet region at the second chamber and metal ions and the sample are injected in the vicinity of the expansion part of the supersonic jet. At the expansion part, the sample and the metal ions collide at a high collision frequency, so there are more opportunities for attachment, the vibration, rotation, and translation motions are cooled, and the surplus energy causing disassociation of the attached ions is quickly removed. In an ion attachment mass spectrometry apparatus, the neutral gas and ions attach to each other, but no Coulomb force is created between the two, so the same situation arises as to the formation of gas clusters between gases. Note that in an inductively coupled plasma mass spectrometry apparatus or atmospheric pressure ionization mass spectrometry apparatus for stripping electrons to obtain positive ions, the supersonic jet does not contribute anything at all to the improvement of the efficiency of ionization.
As a specific condition for forming the supersonic jet, it is sufficient to make the Knudsen number not more than 0.01 and make the second chamber have a pressure of not more than {fraction (1/10)}th of the first chamber. Further, to form a more powerful supersonic jet, it is sufficient to make the Knudsen number not more than 0.001, make the first chamber at least atmospheric pressure, and make the second chamber not more than 1000 Pa.
For injecting metal ions and sample gases in the vicinity of the expansion part of the supersonic jet, three methods for injection from (a) the first chamber, (b) the second chamber side, and (c) a hole in the middle of the nozzle are conceivable. Concerning the injection of metal ions, with injection of metal ions from the first chamber, the high pressure is used to control the flow of gas and transport it to the aperture inlet of the nozzle. In injection of metal ions from the second chamber, the electric field is controlled to control the motions of the metal ions and irradiate the aperture outlet of the nozzle with the metal ions. With injection of metal ions from the middle of the hole of the nozzle, the contact with the expansion part is used for direct irradiation or the inside surface of the nozzle is made the emitter. On the other hand, concerning the injection of a sample gas, with injection of a sample gas from the first chamber, in the same way as the prior art, the sample gas is introduced in the first chamber. With injection of a sample gas from the second chamber, the low pressure (facilitates to inject a sample gas) and, when injecting from the middle of the nozzle, the contact with the expansion part is used for direct introduction.
Using the above method, it is possible to raise the transport efficiency of the metal ions to the attachment region. Further, in every case of (a), (b) and (c), it is possible to raise the transport efficiency of the attached ions to the mass spectrometer by making attached ion stream converge to smaller region, having the attachment region located in the second chamber, and aligning well the speed and direction of ions ejected from these as characteristics of the supersonic jet.
In particular, if satisfying L less than 0.67xc3x97Dxc3x97(P1/P2)0.5 meaning that there is a Mach disk after the aperture provided in the front of the mass spectrometer, the attached ions strike the mass spectrometer while aligned in direction and speed, so a higher transport efficiency can be realized.